Hi,
If you want to search taxonomy terms, but return the posts as results, then make sure that:
– This option is enabled
– And that nothing is enabled under the General Options -> Sources 2 panel: https://i.imgur.com/1JS1G9c.png
You can use the additional keywords to enter the plurals or additional words as you want. Those does not need to be translated via WPML, as WPML creates separate posts for each language. If you edit the translated post, then you can enter the translated keywords into that meta box under the editor screen.